Perception of sound: Localization of sound sources

Let's consider an active sound source and a listener positioned as follows:

Inter-arrival time

The signals that reach the two ears are different to one another. The main differences are:

2.10.1. Time differences (phase)

In the previous figure we saw how the distance of the two ears from the sound source is different. This results in a difference in arrival time, called inter-arrival time, of each signal to the ears (in the practical case shown in the figure, the signal arrives at the right ear first and then at the left ear).

2.10.2. Amplitude differences

The amplitude of the two signals are different both because amplitude decreases with an increase in distance, and because the signal that has to reach the furthest ear has to circumvent the obstacle of the head and in doing so, loses energy.

2.10.3. Harmonic content differences

With reference to the previous figure, we can see that one of the two waves has to "take a detour" round the head to reach the furthest ear. This entails a loss of high frequencies as a result of diffraction
[Diffraction
]
. The higher frequencies won't even be able to go beyond the obstacle thus resulting in a difference in frequency content when entering the ears. This is why it is difficult to identify where low frequency sounds come from: the latter are capable of circumventing obstacles without losing relevant amounts of energy and therefore the sounds that reach the two ears are almost identical. In circumstances when the sound source is right behind the listener, its direction can be identified because a lack of high frequencies is perceived, the latter having been attenuated by the pinna.